You are modeling at very tiny sizes.
SketchUp’s tolerance is set to suit day-to-day objects like TVs and houses,
Not microscopic machine parts.
Anything <~1/1000th inch will fail as the points are deemed to be coincident, so no edge is made and a face needs its edges etc…

That said… tiny geometry can ‘exist’, but it cannot be ‘created’.
So Scale your model up by say x10 [or more], then the FollowMe operations etc will make edges and faces, as you hope…
When you are done Scale back down to the required size.
Then your tiny objects will still exist, because it’s just their initial creation that you can’t achieve !

I ACTUALLY HAVE A MANIFOLD…technically, it isn’t quite perfect but it is good enough to print. I did it at 10x

This is why it isn’t perfect.
When I did the FM on each side then added the end ramp pieces, I made straight lines from the extrusion to the lip of the cylinder.
It creates a flat surface over the curved surface between the 2 extrusions but made a ledge on the other side. Before I realized what was happening I deleted the curved surface - when I should have deleted the flat surface and made both sides the same.

The ? is – is there a way to made a twisted flat face attach to a curved face???

If u can figure out what I am talking about it is 100 points…

picture attached
the bottom is the curved cylinder
the next rectangle is the twisted piece (no face), rotating 90° from rt to lft (or lft to rt).
If the bottom line on that can be made to attach to the curved face than it would be perfect.
This is a picture of the other side and the bottom piece on that is a face to the curve that SU put in itself.